Fluorine-containing cation-exchange membranes are commonly used as a diaphragm for sodium chloride electrolysis, a diaphragm for fuel cells, etc. In order to impart new functions to the membrane, there are known a technique for forming a coating layer containing solid particles (e.g. an electrode powder or alkali-resistant inorganic material particles) and a cation-exchange resin as dispersoids, and a technique for forming a multilayer membrane composed of a large number of layers.
As to a technique for producing a dispersion consisting of a perfluorocarbon-based copolymer, another resin, solid particles and the like, there are known processes comprising melt kneading and melt shaping (see, for example, JP-B-60-44333, JP-A-60-219232 and JP-B-2-19848). According to any of these processes, it is difficult to form a thin layer necessary for imparting excellent functions, on a membrane used as a substrate.
As a technique for forming a thin layer containing solid particles dispersed therein or a technique for forming a multilayer membrane composed of a large number of layers, there is known a technique using a solution or dispersion of a perfluorocarbon-based copolymer as a starting material for the thin layer or the multilayer membrane.
As to a technique using a solution of perfluoro-carbon-based copolymer containing electrolyte type (SO3H type, SO3Na type or the like) functional groups (see, for example, JP-B-48-13333 as to a technique for forming the solution) as a starting material, there are known, for instance, processes for forming a layer of inorganic material particles or electrode powder on a membrane and processes for forming a membrane by impregnating a tetrafluoroethylene stretched porous film with the solution (see, for example, JP-A-59-219487, JP-A-3-137136, JP-A-8-162132, JP-A-8-329962, JP-B-7-103251, U.S. Pat. Nos. 5,547,551, 6,130,175 and 6,156,451). In these processes, since the starting solution is prepared from a perfluorocarbon-based copolymer containing electrolyte type (SO3H type or the like) functional groups, it is difficult to impart a sufficient adhesive strength between resin portions of the perfluorocarbon-based copolymer containing the functional groups. In addition, since the resin structure is changed by a high-temperature history imposed during the preparation of the polymer solution, the structural stability at the time of undergoing swelling and shrinkage and the long-term structural stability and performance stability are low. Moreover, it is difficult to form a multilayer membrane having a high peel resistance on the joint interface between different copolymers or between a copolymer and a substrate. U.S. Pat. No. 6,130,175 discloses that a porous material can be impregnated with a nonelectrolyte type (CO2CH3 type) copolymer, but this technique does not permit sufficient impregnation with a copolymer having a sufficiently high molecular weight which is useful for practical purposes, and merely permits formation of a two-layer structure in which an anchor effect has been imparted to a part of the boundary surface between a porous material and the copolymer, as described, for example, in U.S. Pat. No. 6,156,451.
There has also been disclosed a technique for forming a solution or dispersion of a perfluorocarbon-based copolymer containing nonelectrolyte type functional groups.
There is known, for example, a method in which functional groups are converted to ester type functional groups having a long-chain hydrocarbon-type alkoxy group, such as CO2C10H21 (see JP-B-61-15897). However, according to this method, sufficient melting and strong adhesion to a membrane used as a substrate are difficult because the ester type functional groups (CO2C10H21 type or the like) have such a low thermal stability that the thermal decomposition of said functional groups occurs at a temperature higher than 150 to 180° C.
There is also known a method using a dispersion of a perfluorocarbon-based copolymer containing CO2CH3 type functional groups which is produced from a latex obtained by polymerization in an aqueous medium (see, for example, JP-B-62-59190). However, according to this method, strong bonding among polymer particles and between polymer particles and a substrate membrane by fusion is difficult, for example, because a surfactant remains on the surfaces of polymer particles with a particle size of about 1 μm or less made of the perfluorocarbon-based copolymer. A method is also known in which a solvent in the same latex as above is replaced (see, for example, JP-B-2-10177). However, according to this method, it is difficult to obtain a homogeneous dispersion because the union or aggregation of polymer particles of a perfluorocarbon-based copolymer containing functional groups occurs easily during the replacement of the solvent, so that an inhomogeneous dispersion tends to be formed.
There is also known a technique for producing a solution of a perfluorocarbon-based copolymer having CO2CH3- or SO2F-type nonelectrolyte-type functional groups (see, for example, JP-A-6-192431). However, in this technique, the physical properties of the functional-group-containing perfluorocarbon-based copolymer that is soluble are limited. Therefore, it is difficult to obtain a solution of a functional-group-containing perfluorocarbon-based copolymer having high EW (equivalent weight) value and molecular weight which is useful for practical purposes, and it is also difficult to produce a polymer solution containing a high concentration of a copolymer which is useful for practical purposes.
A polymer solution technique using a fluoro-oligomer as a solvent is also known (see Japanese Patent Application Kohyo No.58-500567). However, this technique does not permit formation of a polymer solution of a copolymer having a sufficiently high molecular weight which is useful for practical purposes.